1. Field of the Invention
The invention is concerned with a process and apparatus for the continuous exchange of aqueous solutions during a chemical or electrolytic treatment on a surface. The invention finds particular, but not exclusive, application to the treatment of narrow surface depressions (such as within a bore) in a large surface or on individual work-pieces or the treatment of a pourable loose mass (charge) of work-pieces of metals or plastics.
2. Description of the Prior Art
It is known that an electrolytic reduction process on cathodic polarized work-pieces proceeds in the very thin liquid layer lying immediately on the exposed surfaces. The thickness of these catholyte films is very small, typically 500 .mu.m with stationary and 10 .mu.m with agitated cathodes (for example, with a relative velocity of 3.25 m/min between the work-piece and the treatment solution). Streams outside this zone have no influence on the reactions in the electrolytically-active boundary layer.
Through reduction, the catholyte becomes poorer in metal ions. The deficiency in ions cannot be balanced out sufficiently quickly through diffusion, natural convection or migration of the electric charge carriers. This deficiency appears more strongly with high current densities.
Operational experience teaches that through an increase in the relative velocity between the surface of the cathode and the surrounding electrolyte the permissible cathode current density and yield rise practically proportionally. The tinning of copper wire can serve as an example. The very rapid passage of a wire through an electrolyte (with a transport speed of 600 to 1300 m/min) causes a massive turbulence on the surface with a correspondingly rapid electrolyte exchange and possible current densities of 32 to 97 A/dm.sup.2.
In the known installations for manufacturing printed circuit boards having track-connecting through-holes, the boards are moved at a low frequency of about 30 oscillations per minute and with an amplitude of 20 to 60 mm horizontally, in order to bring about the necessary electrolyte exchange in the drilled holes and at the same time to remove hydrogen bubbles. The holes between the conductor tracks generally have a very small diameter (for example from 0.8 to 1.2 mm) and may be present in large numbers (for example 25,000 to 90,000 drillings per m.sup.2). The development tendency in the production of conductor tracks is constantly towards narrower conductor paths, smaller distances between conductors, smaller connecting through-holes and thicker boards. A board of 7.3 mm thickness with drilled holes of 0.35 mm diameter, and thus with a hole ratio of 21:1 can serve as an example.
In view of the very long work-piece linear movement paths, the low speeds and the gentle reversal processes in the known installations, it is obvious that the electrolyte quantities, confronting the large area flat boards, in fact oscillate together in the same rhythm with the boards and thus the relative speeds come to a condition which proves not to be sufficiently large for an effective electrolyte exchange, in particular in the drilled holes. For this reason it has been proposed to intensify the hole treatment with a forced through-flow generated by ultrasonics. Resorting to ultrasonics means a reversal in principle of the previously described systems generally employed, in order to obtain an electrolyte exchange on the surface of the work-piece. In this way, the fluid boundary layer is set in motion against the surface of the work-piece. The frequency of the ultra-sound lies above a threshold of about 10 kHz. It is known that in electrolytic metal-deposition, ultra-sound fields cause a strong agitation of the electrolyte in the fluid boundary layer, and through this bring about a rapid concentration balance. However, the use of ultra-sound in metal deposition on conductor boards breaks down principally because the very thin (copper) layers at first chemically deposited on the surfaces of the synthetic conductor-boards (from about 0.35 .mu.m to about 8 .mu.m thick) only adhere thereto in a limited way and a liquid put into oscillation during the tension-phase of the sound-wave--depending on the frequency, amplitude and intensity--leads to the tearing away of the already reduced copper layer from hollow spaces (cavitation effects) on the board surfaces. These effects occur in particular when--as an accompanying effect of the reduction process--hydrogen bubbles appear.
Homogeneous, sufficiently exactly controllable sound-fields in liquids can be established only with an extraordinarily high apparatus cost; installations with ultra-sound-boards and dipping oscillators are therefore hardly available for commercial practice.
A similar process for the electrolyte exchange, and in fact to move the treatment solution against the conductor boards and not the other way round, has become known as a `horizontal through-flow process`. The individual solution containers are closed with a cover. The conductor boards are transported horizontally from one treatment station (tank) to the next: during the stay of a board in the individual stations the treatment solution is pumped out of a storage container and forced through flow-pipes on to the horizontally moving conductor boards. This leads to an intensified electrolyte exchange, in particular giving a forced flow through the holes in the conductor boards. Finally, the solutions flow by way of a catchment tank back again into the storage container.
In comparison with the conventional installations, which treat the conductor boards in a vertical position, the installations in the horizontal through-flow process are correspondingly longer and effectively more complicated in their technical installation construction. Their defects are obvious: greater space requirement, higher installation costs, increased electro-mechanical failure liability and corresponding maintenance costs.
The horizontal system also excludes the treatment of the conductor boards in a so-called batch process, in which a parallel arrangement of a number of conductor-boards on a single structure go through the whole treatment process simultaneously.
A problem similar to that which arises with the conductor boards occurs in the surface treatment of a mass of pourable parts in a dipping drum. The continually decreasing diameter of the drill holes in the conductor boards and their greater packing density lead inevitably to the diameter of the inserts for the tracks becoming ever smaller. If such components are provided also with tube-like recesses or blind holes, then these can hardly be chemically or electrolytically treated with the known dipping drums rotating about their longitudinal axes. Apart from lack of solution exchange in the walls of the bores, there often occurs the condition that through an insufficient capillary action, there is no solution at all in the bores.
Generally, a drum has a regular geometrical cylindrical form and turns about its horizontal axis of rotation, which is also at the same time its longitudinal and symmetrical axis. From the standpoint of the flow mechanics the rotating drum can be regarded as a hydraulically smooth body, which means that hardly any flow of the electrolyte takes place through the perforated drum wall. It is similarly known that the compact irregularly assembled mass of the drum-load electrically resembles a Faraday cage and that the electrolytic reduction process proceeds only on the periphery of the load. The deficient solution exchange inside the load leads additionally to the so-called depth distribution of the electrolyte reducing similarly, and the mass of parts is unequally treated with a correspondingly high reject rate.
The advantages of causing vibration oscillations in the electrolytic surface treatment of mass parts and a number of devices for making use of this process are known. The constant relative movement of the load as a whole caused by the vibration against the surrounding electrolyte proceeds continuously to all individual parts in the mass with a similar intensity, irrespective of their instantaneous position inside the load.
The known appliances consist in effect of a circular, horizontally supported plate-like dish, open above and dipping into the electrolyte, which receives the load, there being a centrally-supported vertical bearing-shaft on the dish, and a vibrator with its support structure. The dish and the heavily-loaded vibrator are on the vertical extremities of the device. The bearing-shaft transfers both the oscillations from the vibrator and also the electric current to the load in the dish. A cylindrical wall surrounds the spirally-formed base of the dish, in order to prevent the mass parts from falling out. The dish does not execute any rotational movement about the support shaft. The vibrator produces a throwing movement for the load mass, in that it oscillates the dish reversibly both vertically and also at the same time about the support shaft as a center (forward-upward and back-downward). The forward and backward processes of the diagonally upward directed throwing movement have the same time duration. Through this, the load lying on the dish is set into a flowing movement: it climbs--jumping--up the spirally formed bottom of the dish as a ramp, then falls down thoroughly mixed from its radially directed edge.
In view of this, the charge circulates, jumping as a whole unit, without becoming mixed together, circularly about the support column of the round dish: the mixing process takes place only locally and over an extremely short time on the edge of the ramp.
The previously described thorough mixing process of the load is one of the decisive disadvantages of the known equipment. The individual mass parts change their relative positions only very little; an individual part moves much more in relatively equal circular paths about the central column as a middle point. The mass parts in the peripheral plate-circumference of the load experience in this a considerably higher cathodic current density (as a result of their shorter distance to the anodes) than those which circulate in the central region of the support shaft. The electrolytic deposition consequently takes place unevenly on the load as a whole. In order nevertheless to arrive at a layer having as even as possible thickness distribution, only very low plating currents are permissible: this circumstance leads inevitably to unusually long, uneconomical treatment times.